The present invention relates to a tubular acoustic attenuation piece for an aircraft jet engine air intake.
It is known that jet engines with which aircraft are fitted are sources of noise at a very high level (often in excess of 130 decibels). That is particularly troublesome if a jet engine is located near to an inhabited area, for example when an aircraft takes off or lands. Environmental standards, which are becoming increasingly strict, dictate that the level of noise from a jet engine be limited to below a regulation threshold so as to limit the acoustic disturbance perceived by the inhabitants of said area. This regulation threshold is sometimes lowered still further during the night-time, which means that the noisiest aircraft can neither land nor take off, and this is a constraint, or even a loss of potential income, for the airlines which operate such aircraft. In consequence, it is desirable to reduce the level of acoustic emissions of such a jet engine to below said lowered regulation threshold so as to be able to operate at any time of the day or night.
In the known way, the level of noise emitted by an aircraft jet engine can be reduced by arranging, on the interior surface of the air intake of said jet engine, a cellular-core sandwich structure tubular piece, of which the face facing the inside of said air intake is formed of a permeable material allowing air to pass between said air intake and the cells of said cellular-core sandwich structure so that these cells can trap some of the sound waves and thus reduce the level of noise emitted to the outside by the jet engine. However, for ease of construction and fitting, this cellular-core sandwich structure piece is often made as several shells, each of which forms a longitudinal part of said tubular piece and which are assembled by means of longitudinal fish plates which join the facing lateral edges of adjacent shells together.
Such fish plates generally comprise a longitudinal fish plate strip arranged on the external side of said tubular piece and a longitudinal fish plate strip arranged on the internal side thereof. The internal longitudinal fish plate strips constitute areas of the interior surface of the air intake in which, on the one hand, the noise is not absorbed and, on the other hand, the sound waves are reflected, thus having the effect of greatly degrading, in terms of noise level reduction, the performance of said piece by comparison with a monolithic piece.
Furthermore, as can be seen for example in the documents of the prior art U.S. Pat. No. 4,840,093, U.S. Pat. No. 4,969,535, U.S. Pat. No. 5,014,815, U.S. Pat. No. 5,025,888 and EP-1 167 185, the lateral edges of the adjacent shells for securing are configured in a special way, for example thinning them continuously or stepwise, or inserting a densified cellular core there, etc. In any case, these configurations are lengthy, tricky and expensive.
Starting out from the principle that attempts made hitherto at obtaining acoustic insulation which is as undegraded as possible at the longitudinal connections between said adjacent shells are relatively in vain and that they lead to high costs, the object of the present invention is a multi-shell tubular acoustic attenuation piece that is simple, easy to manufacture and also not only leads to acoustic degradation that is no greater than that caused in the known multi-shell pieces but is also entirely comparable, as far as acoustic performance is concerned, with monolithic pieces. To this end, according to the invention, the tubular acoustic attenuation piece for a jet engine air intake, comprising:
at least two shells, each of which forms a longitudinal part of said tubular piece and which are connected along their longitudinal edges to form said piece, the wall of each of said shells consisting of a permeable internal skin, of an impermeable external skin and of a cellular core inserted between said internal and external skins; and
for each connection between two longitudinal edges of two adjacent shells, an external longitudinal fish plate strip arranged on the side of the external skins of said adjacent shells, straddling said connection, and secured to said external skins,
is notable in that:
said external fish plate strips alone secure said shells together and are fixed to the latter by fixing means which are not visible on the side of said internal skins;
each of said longitudinal connection edges consists of an edge face of said corresponding shell, along which edge face the cellular core of this shell has a marginal area of the same thickness as the rest of said cellular core but which can be damaged, so that said shell, corresponding with said marginal area, has acoustic attenuation which is not as good as in the remainder of said shell;
two facing longitudinal edges of two adjacent shells leave a slot between them; and
the sum of the width of said slot and of the widths of the two facing marginal areas is at most equal to 15 mm.
The present invention is based on the experimental observation by the applicant company who found that, provided there is no projection (fish plate, fixing means, etc.) on the internal skin of said shells and provided that an upper limit (15 mm) on said sum of the widths is adhered to, it was possible, regardless of the jet engine concerned, to obtain good acoustic attenuation with longitudinal connection edges which are assembled with clearance (the slot) and along which said cellular core is simply cut (which opens up cells of said core and creates damaged marginal areas with reduced attenuation capability).
Thus, the longitudinal connection edges of the tubular acoustic attenuation piece according to the present invention entail no lengthy, tricky and expensive preparation such as gradual or stepwise thinning for example or partial replacement of the cellular core with a denser cellular core. The costs of manufacture of the tubular piece according to the invention are therefore reduced to a minimum.
On the inside of said tubular piece according to the present invention, the internal fish plate strips can be eliminated and the only things visible there are therefore said slots, the narrow width of which may just correspond to a mounting and operating clearance.
Said means for fixing said external fish plate strips may be blind bolts passing only through said external skin and partially housed in said cellular core. As an alternative, they may consist of adhesive.
In order to reduce the effect of the presence of said narrow slots on the inside of the tubular piece according to the invention still further, it is advantageous for these to be filled, at least on the inside, with a flexible seal, for example made of polysulfide mastic.
As a preference, to avoid delamination of the edges of said shells, said longitudinal connection edges (the edge faces) are covered with a folded continuation of at least one of said internal and external skins.
Since, in order to be able to eliminate the internal fish plate strips and their acoustic inconveniences, it is necessary for said external fish plate strips to be able by themselves to secure said shells together, said external fish plate strips need to allow connections to be made the firmness and rigidity of which are such that the loadings applied to said tubular piece do not cause deformation of the connections that is appreciably greater than the deformation that would occur if said tubular piece were monolithic.
To this end, said external fish plate strips, which are, for example, made of metal, may, on their opposite external face to said external skins, carry stiffening means, such as individual transverse stiffeners or longitudinal stiffening strips at least partially covering said external fish plate strips. Such stiffening strips may comprise at least one cellular core so as to form a sandwich composite structure able possibly to attenuate the sound waves passing through said connecting slots.
It is advantageous for said external skins to be thicker than said internal skins so as to transmit the high loads to which said tubular piece is subjected. In addition, the parts of the external skins of said shells to which said external longitudinal fish plate strips are fixed may be reinforced.